Public issue https://github.com/mackron/miniaudio/issues/341

  * If starting the backend fails, don't attempt to enter the data loop
  * If stopping the backend fails, don't fire the stop callback.

Public issue https://github.com/mackron/miniaudio/issues/344

Public issue https://github.com/mackron/miniaudio/issues/341

This was happening because ma_log_level_to_string() was only being
included when MA_DEBUG_OUTPUT is enabled.

Fix `ma_log_postv` not returning anything under certain circumstances

The `__STDC_VERSION__ >= 199901L` and the `_MSC_VER >= 1200` code
had different styles for handling errors, so I've made them match.

MSVC 6 was printing a warning about this.

Public issue https://github.com/mackron/miniaudio/issues/340

Fix invalid vsnprintf buffer size

Was performing a `sizeof` on a pointer, which causes a warning to be
generated.

This fixes an issue where excess channels are not heard when the sound
source has more channels than the listener.

This commit also fixes a bug where panning is incorrectly getting
applied to non-directional channels, such as mono and LFE channels.

This is work towards using the heap for storing per-channel data so we
can get rid of upper channel count limit and remove MA_MAX_CHANNELS or
at the very least stop MA_MAX_CHANNELS from affecting overall memory
usage.

Previously this was restricted to 2 input buses and 2 output buses, but
this has been lifted to 254. When the number exceeds 2, internal data
structures will be allocated on the heap, otherwise they'll use a local
array contained within the ma_node structure.

This commit changes the node configuration. Previously there was a
fixed sized array for specifying the channel counts for each bus. This
array must now be defined outside of the config by the caller. The
following config variables have been renamed:

  * inputChannels > pInputChannels
  * outputChannels > pOutputChannels

This commit also adds the ability to configure input and output bus
counts on a per-instance basis rather than via the node vtable. To do
this, set the bus count in the vtable to MA_NODE_BUS_COUNT_UNKNOWN.
This will tell miniaudio to look at the node config to determine the
bus count rather than the vtable. It's an error to specify this in the
node config if the vtable specifies anything other than
MA_NODE_BUS_COUNT_UNKNOWN.